1. Field of the Invention
The present invention relates to a trickle-development type developing device which uses a two-component developer and an image forming apparatus which includes the developing device.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner to the photoconductor (image carrier) on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet through an intermediate transfer belt, followed by heating and pressurization for fixing, whereby an image is formed on the sheet.
Development methods for forming a toner image on a photoconductor include the one-component development method which uses only toner as the main component of a developer, and the two-component development method which uses toner and carrier as the main components of a developer. In the two-component development method, toner and carrier are mixed and stirred to frictionally charge the toner. Ideally, to stably charge the toner, the surface of the carrier does not change.
In a two-component development type developing device, toner is consumed in a development process, while carrier is not consumed and left in the developing device. Therefore, the carrier accumulates mechanical stress and thermal stress due to the contact with the toner, and the surface of the carrier is contaminated by the attachment of the toner. Under such circumstances, the trickle-development method has been widely used in which carrier is periodically replaced by outputting the developer (developer to be discarded) containing degraded carrier while newly supplying a developer (toner and carrier).
In such a two-component development type developing device, for example, the toner concentration (the ratio of the toner to the total amount of the developer) in a developer container is determined by a toner concentration sensor, and toner is supplied so that the toner concentration falls within a desired concentration range, that is, toner is supplied in accordance with a toner consumption amount (see, for example, Japanese Patent Application Laid-Open No. 2005-292376 (Patent Document 1)). In Patent Document 1, an operation for supplying a developer in which toner and carrier are mixed at a certain ratio is controlled on the basis of the toner concentration. Typically, the toner concentration sensor is provided at a remote position on the downstream side relative to a developer supply port in the developer conveyance direction, and near a bottom of a developer container in order to detect the toner concentration in a stable state in which toner and carrier are sufficiently stirred.
On the other hand, in the case where carrier is supplied together with toner in accordance with the toner consumption amount, the following problems may be caused. Specifically, when the toner consumption amount is large, even non-degraded carrier is replaced, and thus the carrier is wastefully discarded. When the toner consumption amount is small, the degraded carrier is used without being replaced. As a result, the charging performance is degraded, degrading the image quality. Under such circumstances, there is a developing device in which a toner supplying section and a carrier supplying section are separately provided, and toner is supplied on the basis of the toner concentration, while carrier is periodically supplied at predetermined time intervals.
In the case where carrier is supplied together with toner as is the case of the developing device disclosed in Patent Document 1, whether an operation for supplying developer has been normally performed can be determined on the basis of results of detection obtained by the toner concentration sensor. However, in the case where toner and carrier are separately supplied, it is difficult to detect by the toner concentration sensor whether the carrier supplying operation has been normally carried out. The reason for this is that, typically, the amount of carrier which is supplied by the carrier supplying operation is small, and the carrier concentration (100-toner concentration[%]) is not substantially changed in a stable state where toner and carrier have been sufficiently stirred.
Therefore, when carrier is not normally supplied and the supply of carrier is stopped, the degradation of carrier is facilitated in the developer container and the amount of the carrier becomes insufficient, and consequently, the image quality may be degraded due to fogging, toner scattering and the like.